Orthogonal multiple access (OMA), in which multiple signals do not interfere with each other, is widely used in communication between a base station and user equipments (e.g., mobile stations) in a mobile communication network. With orthogonal multiple access, different radio resources are allocated to different user equipments. CDMA (code division multiple access), TDMA (time division multiple access), and OFDMA (orthogonal frequency division multiple access) are examples of orthogonal multiple access. For example, in Long Term Evolution (LTE) standardized by the 3GPP, OFDMA is used in downlink communication. With OFDMA, different frequencies are allocated to different user equipments.
In recent years, non-orthogonal multiple access (NOMA) has been proposed as a method for communication between a base station and user equipments (e.g., see Patent Document 1). With non-orthogonal multiple access, the same radio resources are allocated to different user equipments. More specifically, a single frequency is allocated to different user equipments at the same time. In the case of applying non-orthogonal multiple access to downlink communication, a base station transmits a signal with a large transmission power to a user equipment (commonly a user equipment at a cell area edge) with a large path loss, i.e., a user equipment with a small reception SINR (signal-to-interference-plus-noise-power ratio), and the base station transmits a signal with a small transmission power to a user equipment (commonly a user equipment at the center of a cell area) with a small path loss, i.e., a user equipment with a large reception SINR. Accordingly, the signal received by each user equipment is influenced by interference caused by signals addressed to other user equipments.
In this case, each user equipment demodulates the signal addressed to that user equipment using a power difference. Specifically, each user equipment first demodulates the signal with the highest reception power. Because this demodulated signal is a signal addressed to a user equipment closest to the cell area edge (or more accurately, the user equipment with the lowest reception SINR), the user equipment closest to the cell area edge (the user equipment with the lowest reception SINR) ends demodulation. Each of the other user equipments cancels out the interference component corresponding to that demodulated signal in the received signals using an interference canceler, and demodulates the signal with the second-highest reception power. Because this demodulated signal is a signal addressed to a user equipment that is the second-closest to the cell area edge (or more accurately, the user equipment with the second-lowest reception SINR), the user equipment that is the second-closest to the cell area edge (has the second-lowest reception SINR) ends demodulation. By thus repeating the demodulation and canceling out of signals with high power, all of the user equipments can demodulate the signals addressed to them.
By combining non-orthogonal multiple access with orthogonal multiple access, it is possible to increase the capacity of the mobile communication network in comparison to using orthogonal multiple access alone. That is, in the case of using orthogonal multiple access alone, it is not possible to allocate a certain radio resource (e.g., a frequency) to multiple user equipments at the same time. In contrast, in the case of combining non-orthogonal multiple access and orthogonal multiple access, a certain radio resource can be allocated to multiple user equipments at the same time.
MIMO (Multiple Input Multiple Output) is used in mobile communication networks. In MIMO, precoding is performed at a base station in order to perform multi-stream beamforming thereat.